There is a strong need for systems and methods for measuring water vapor concentration in the fields of atmospheric science, agriculture, and industry such as the microelectronics, steel, textiles, paper and food industries. Water vapor concentration is typically measured using either dew point sensors or relative humidity sensors. The relative humidity (RH) and the dew point of a gaseous sample are closely related by well known psychometric algorithms for converting dew point and ambient temperature to relative humidity or converting RH and ambient temperature to dew point. The dew point is defined as the temperature at which the partial pressure of water vapor is equal to the vapor pressure of water.
Commercially available dew point sensors such as chilled mirror hygrometers utilize complicated and cumbersome methods to detect the presence of dew on a chilled surface. The detection scheme consists of directing a light beam using a diode laser onto a mirror which reflects the light beam onto a photo detector. While the mirror is dry the reflective characteristics of the mirror are good and a substantial amount of light is received by the photo detector. With the presence of dew on the mirror, however, the reflective characteristics of the mirror change and the light which reflects off the mirror is diffused, thus reducing the amount of light reaching the photo detector. The mirror is in thermal contact with a chiller and is capable of being cooled to below the dew point so that the dew can form on the mirror surface and the temperature at which the dew forms is by definition the dew point of the ambient environment.
A temperature sensor in thermal contact with the mirror provides the measurement of the mirror temperature at the dew point. Therefore by monitoring the amount of light detected by the photo detector, the temperature at which dew forms on the mirror may be determined. While this method is accurate, it is cumbersome and expensive to implement and maintain; for example, contamination of the mirror surface may lead to inaccurate readings or a malfunction of the sensor.
Other prior art sensors such as that disclosed in "Improved Dew point Measurements Based on a SAW Sensor", by Vetelino et al. in Sensors and Actuators, B35-36(1996), have proposed improvements to the chilled mirror hygrometer. Such prior art solutions utilize surface acoustic wave (SAW) devices to detect the presence of moisture. A SAW sensor generates surface acoustic waves which are transmitted through a delay path using the reverse piezoelectric effect. When the sensor is cooled, condensation forms on its surface which attenuates the wave and alters its velocity. The dew point may then be determined by monitoring the changes in the wave velocity, frequency or phase. This method offers some advantages over the chilled mirror system; however, SAW devices are extremely sensitive to a multitude of physical effects, parameters and variables such as temperature, pressure, and vibration. Isolation or compensation for these parameters is therefore necessary to achieve satisfactory performance of these devices which results in increased complexity and cost.
Another prior art solution in U.S. Pat. No. 4,877,329 (Sauerbaum et al.) discloses a sensor which is chilled to form condensation thereon. The condensation is detected by a comb-like interdigited sensor which outputs a varying electrical signal in response to variations in the thickness of the formed dew layer. For example, the varying electrical signal may be a function of a change in capacitance of the sensor due to a varying amount of moisture in the dielectric caused by variations in the amount of dew. Along with the sensor structure, Sauerbaum et al. teach a method for measuring the dew point that includes cycling a heating and cooling device to maintain a constant layer of dew at the measured dewpoint and a step for calibrating the sensor to avoid errors by displacement of the zero point due to, for example, contamination. While this device and method improve the cost and complexity of a dew point measurement system over a chilled mirror hygrometer, the device is subject to errors and drift in the interdigited sensor material. These problems are well understood to be inherent in the sorption type humidity sensors and a property of the materials commonly chosen to adsorb the moisture.
A related group of prior art sensors measure the relative humidity of an ambient environment as opposed to the dew point. As discussed above, relative humidity and dew point are easily converted from one to the other with a measurement of the ambient air temperature. As disclosed in "Micromechanical Sensors for Chemical and Physical Measurements", by E. A. Wachter et al., in Rev. Sci. Instrum. 66(6), June 1995, a relative humidity sensor utilizes a micro-cantilever beam (MCL) with a sorption coating of gelatin for measuring humidity. No chilling of the environment is performed since the dew point is not being measured; instead, the MCL's coating changes mass as more water is absorbed at higher humidity levels. This mass change is detected by the shift in a resonant frequency of the MCL. While incorporating a sorption sensor onto a micro-electromechanical device would conceivably give a better sensitivity than a standard sorption type sensor, the prior art unit is still plagued by the common problems of sorption type sensors including hysteresis. For example, if the relative humidity of the environment is 50% with a corresponding MCL mass of X, and the relative humidity increases to 90% with a corresponding MCL mass of Y, a subsequent decrease in the relative humidity back to 50% will not result in the MCL mass decreasing back to X, but rather the mass of the MCL will decrease to X+.DELTA.X, wherein .DELTA.X corresponds to an error due to the hysteresis of the sorption film.
Similar to Wachter et al., U.S. Pat. No. 5,563,341 (Fenner et al.) teaches a vapor pressure sensor which utilizes a coating on a restrained or non-vibrating cantilever beam. Changes in vapor pressure of a selected material within the ambient cause stress in the coating and beam which is measured to provide an indication of the vapor pressure. Again, this prior art method offers sensitivity advantages over standard sorption sensors but still suffers from the limitations of these coatings such as hysteresis.
In addition to the undesirable hysteresis impacts of sorption films, sorption materials are typically polymer films which corrode over time. Furthermore, over the lifetime of the film the moisture within the film evaporates which results in the film "drying out", thus resulting in further film degradation which cause variations in its sorption properties, thus further contributing to measurement errors.
There is therefore a need in the art for a dew point sensor having a reduced cost and complexity and which does not suffer from errors inherent in sorption type sensors.